2,103. Even 1,000 Days
Hilchos Issurei Biah 8:21
Even if a woman’s period were to last for 1,000 days, as soon as she stops bleeding, she should count seven clean days. After these seven days, a woman who made a mistake regarding her days starts her niddah days all over again.
Hilchos Issurei Biah 8:22
Pursuant to the last few halachos, if a woman errs regarding her days, she never counts fewer than seven nor more than 17 days from the time her bleeding stops, which is followed by her niddah days. Accordingly, if she saw blood for one day and it stopped, she counts 17 days: six to complete her niddah days and 11 as her zivah days. If she sees blood for 13 or more days, she counts seven clean days after her bleeding stops, followed by her niddah days, as we discussed in the previous halacha.